1. Field of the Invention
This invention relates to ophthalmic instruments, and more particularly to an improved photokeratoscopic method and apparatus for measuring corneal curvature and topography.
2. Description of Related Art
In recent years, there has been increased interest in instruments that provide a quantitative measurement of corneal topography. This interest is due mainly to developments in surgical procedures that correct refractive errors through modification of the corneal shape. These procedures include radial keratotomy, epikeratophakia, and excimer laser keratectomy.
The most common method of assessing corneal topography is keratoscopy, which involves imaging a pattern of bright concentric circles of light (typically about 20) by reflection from the anterior surface of the cornea. The image of the circles may be interpreted qualitatively, with abnormalities in corneal shape producing a non-circular image. If the images are captured on film or by video imaging, the method is known as photokeratoscopy.
FIG. 1 diagrammatically shows a conventional photokeratoscope. An eye 1 is positioned external of an instrument which comprises (1) a ring generator 3, which may be planar, spherical, or conical, configured to project bright rings of known dimension upon the eye 1 when illuminated from the opposite side of the eye, (2) a light source 5, which is preferably toroidal so as to provide relatively uniform illumination behind the ring generator 3, (3) a lens group 7 for focusing the reflected images of the projected rings received from the eye, and (4) an imaging device 9 for capturing the focused image from the lens group 7. In most modern instruments, the imaging device 9 would digitize the focused image so as to provide a digital image that could be automatically evaluated or provide quantifiable information with the assistance of human intervention. The imaging device 9 may, for example, use a 512.times.512 pixel CCD device, which are commonly available from a number of commercial sources.
FIG. 2 is a close up of an eye 1 and a ring generator 3, showing that ribs or structures 11 around the ring generator (a cone in the example shown in FIG. 2) cause a series of bright light rings 13 to be projected onto the cornea of the eye 1.
Attempts have been made to quantitatively interpret photokeratoscopic images by digitizing the light ring images reflected from a cornea, and using selected equations implemented as computer programs to calculate corneal topography from the size and shape of the light ring images. However, exact calculation is not possible from photokeratoscope data, and many assumptions are made to obtain an estimate of the corneal topography.
A number of references have described such methods. One such reference is entitled "Improved Method for Calculation of Corneal Topography for any Photokeratoscope Geometry", by Paul P. van Saarloos and lan J. Constable, Optometry and Vision Science, Vol. 68, No. 12, pp. 960-965, 1991, the teachings of which are hereby incorporated by reference. The van Saarloos reference teaches a mathematical method for estimating the central corneal radius of curvature and for calculating corneal topography from the radii of the rings in a photokeratoscope image.
FIGS. 3A and 3B are diagrams of the geometry of a conventional photokeratoscope in use with respect to a cornea C. The relevant angles and lengths are indicated, and have the meanings defined in the van Saarloos reference. The van Saarloos equations give a reasonably good approximation of the central corneal radius of curvature and corneal topography by using the measured radii of the rings in a photokeratoscope image and applying the known geometry of a photokeratoscope.
However, one problem with the van Saarloos method and other prior art methods is that, the distance d from the photokeratoscope lens to the apex of the cornea cannot be measured directly. The distance d is used in all equations to calculate corneal topography. Hence, an inaccurate value for d will affect the accuracy of all topographical calculations. The magnification (an hence angle a.sub.i) is also highly dependent on the actual value of d.
The distance d can be approximated if the working distance wd (defined as the distance from the lens group 7 to the plane where an object would be perfectly focused onto the imaging device 9) and the photokeratoscope is focused perfectly. Put another way, d cannot be calculated accurately from wd if the photokeratoscope is not focused perfectly. Unfortunately, achieving perfect focus is often very difficult with present photokeratoscopes.
Accordingly, what is needed is a measurement of the distance d at the time a photokeratoscopic image is recorded. The present invention provides an improved apparatus and method for measuring d, which permits better accuracy than the prior art in calculating the central corneal radius of curvature and corneal topography.